MEG II 実験のための 陽電子タイミングカウンター実機建設 Construction of Positron Timing Counter for MEG II experiment 西村美紀（東大） 他 MEGIIコラボレーション 日本物理学会 2015年 秋季大会 大阪市立大学（杉本キャンパス）

Contents Introduction Physics Motivation MEG II Experiment Positron Timing Counter Status Counter Mass Production Scintillator SiPM Counter property Summary 2

Physics Motivation 3 One example

4 What should we measure? 4 We should measure Timing・Position・Momentum precisely. Accidental BG Dominant Physics BG Signal Background (Michel decay) The resolutions of every detector improved twice.

5 5 Drift Chamber Positron Timing Counter Liquid Xenon Gamma-ray Detector Superconducting Magnet Muon Positron Gamma-ray Radiative Decay Counter MEG II

6 Overview of Timing Counter 4 cm or 5 cm 12 cm Thickness 5 mm 6 SiPMs in series at the both ends AdvanSiD (Italy) 3x3 mm 2, 50x50 um 2 pixels Fast Plastic Scintillator BC 422, rise time 0.35 ns attenuation length 8 cm With reflector (3M film) PCB Cable （ RG 178） Non-magnetic PCB Back plane Long PCB ~ 80 cm Multi layer, coaxial like Optical Fiber for laser light Inter-counter time offset is calibrated by laser. 256 x 2 (up and down stream) counters Next talk M. Nakao 6

7 Multiple Hit Since TC consists of many small counters, the positrons hits many counters. Number of hit counters (MC) *In magnetic field 7

Expectation of Overall Resolution 8 Resolution vs. # of hit counters σ < 30 ps Resolution (psec) ~ 5 ps ~ 30 ps Multiple- scattering Inter-counter jitter from electronics chain * Path length is corrected by drift chamber track information

9 9 Up to the Present Single Counter R&D Study basic property Optimize the counter design Construction Multi Counter R&D Prove multiple hit scheme Check rate dependence Software development Reconstruction Simulation Calibration Laser Muon Normal Decay Engineering run Beam Single Counter Test Next talk M. Nakao Next next talk K. Yoshida 9

Engineeri ng run Schedule We will construct positron timing counter with SiPMs in this year. Engineering run with half counters of TC down stream will start in this November to confirm the final performance in the MEG II environment. 10 Aug. Downstream Construction Upstream Engineering run and physics run Oct. Dec. Feb. Apr. Jun counters

Counter Mass Production Goal for this year: 128 counters by middle of October. SiPM Array Scintillator Single SiPM Counter production Counter Test Scintillator Saint-Gobain BC plates 360: 120x40x5 mm 3 240: 120x50x5 mm 3 SiPM AdvanSiD, ASD-NUV3S-P High-Gain (MEG), 3x3 mm 2,50x50μm 2 pixels 3248 SiPMs (correspond to 541 arrays) SiPM array 6 SiPMs in series connection 11

Scintillator Test with a SiPM array on one side of scintillator. Check the light yield of plates by monitoring current of SiPMs. 3/4 scintillators are worse than we ordered. We will replace it. The company already started new production for 3/4. 12 Resolution (ps) Current of SiPMs (μA) 5 cm result Different colors correspond to different batch. The same as prototype Pico ammeter Measure the current

SiPM Single IV curve Break down voltages are stable. (RMS 0.03 V) However IV curve behavior is not the same. We see two type of IV curve. One brows up faster. Type-2 seems to have larger dark current. (45.5 % is the type-2) 13 Type -1 Type -2

SiPM dark current and production Some group have lower dark current, but that in the other group spread large range. We use all of them even though some SiPMs have high dark current. We selected similar dark current 6 SiPMs for one array. 14

SiPM array 15 6 SiPMs are connected in series (= array). We got 541 arrays. 16 arrays cannot be used. 12 arrays: Window become yellow because soldering temperature is high. 1 array: One SiPM window is broken. 1 array: We didn’t see any signal. 2 array: IV curve is strange. yellowed normal

SiPM array selection 16 We rejected 24 arrays by comparing IV curve with single SiPM IV. 21 arrays: current is inconsistent with single SiPM measurement. 3 arrays: break down voltage is inconsistent with single SiPM measurement. Current of arrays 3.0 OV Expected current from single SiPM 3.0 OV for 4 cm 0 1 BD Difference (V) BD Difference b/w single and array meas

SiPM array 17 4 cm 5 cm Prototype (4 cm) Relative PDE with scintillator light PDE of final SiPMs are worse than one of prototype. Check IV curve They also have two type of IV behavior. 2 arrays are strange.

Optimal voltage If IV curve blows up fast, optimal OV range becomes narrow because of dark current. 18 Final counter with Type -1 SiPMs Final counter with Type -2 SiPMs Bias scan with type-1 and type-2 counter

Counter Mass Production Make combination with similar property (= dark current and relative PDE) arrays. With Optical cement (BC600), we attached SiPM arrays to scintillator. We made 138 counters. Remove void in vacuum 19

Counter Mass Test Check the resolution for every final counter. (measure at 3 points) Under the way; now we finished 62 counters. 20 Source Sr90 (<2.28MeV, β-ray) Digitizer （ 1.6GHz sampling speed ） Reference Counter 5x5x5 mm (BC422), 1 Hamamatsu SiPM Trigger, Collimator Shaping Pole Zero cancellation Test Counter degree

Counter Mass Test Final counter resolutions are ps. With prototype we expected 64 ps resolution.  Overall resolution becomes bad 15 % Still TC overall resolution is ~30 ps. This difference come from low PDE. SiPM production Soldering process under investigation. 21 w/ prototype SiPMs *Not with all scintillator Final counters The difference is 10 ps.

Support structure For this year run we have only produced part of the full TC DS structure (50% ) 16 backplanes (final layout) has been produced for this year run. 22

Summary Positron timing counter based on segmented scintillation detector with multiple-SiPM readout has been developed for MEG II. The construction of TC is on-going. Scintillators and SiPMs tests were curried out. Light yield of 3/4 scintillator seems small, but they will be replaced. SiPMs PDE are smaller than the prototype one. All counters for engineering run were produced. Counter resolution is ps. Though the resolution of the final single counters are worse than one of a prototype counter, overall resolution of ~30 ps is still expected. 23

Back up 24

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Reminder (5 cm) 3.1 OV

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